Three-dimensional phase-field modeling of spinodal decomposition in constrained films

The morphological evolution during spinodal decomposition in binary alloy thin films elastically constrained by substrates is studied. Elastic solutions, derived for both elastically isotropic and anisotropic thin films subject to mixed stress-free and constraint boundary conditions, are employed in a three-dimensional phasefield model to investigate the effect of coherency strain and substrate constraint on microstructural evolution. The temporal evolution of the Cahn-Hilliard equation under thin film boundary conditions is solved with a semi-implicit Fourier-spectral method. The phase separation with coherency strain in an elastically anisotropic film shows the behavior of surface-directed spinodal decomposition driven by the elastic energy effect. Negative elastic anisotropy in the cubic alloy causes the alignment of the phases along <100> elastically soft directions. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Effect of substrate constraint on spinodal decomposition in an elastically inhomogeneous thin film

Spinodal decomposition in a binary thin film is studied by a three-dimensional (3D) phase field model. A cubic thin film with an (001) orientation is considered. The focus is on the effect of the types of substrate constraint on the morphological evolution during spinodal decomposition as compared to the corresponding bulk. The elastic strain effect is incorporated by solving the elasticity equations for an elastically inhomogeneous thin film with a free surface and constrained by a substrate. Temporal evolution for the composition field, and thus the morphological evolution, is obtained by solving the Cahn-Hilliard equation using a semi-implicit Fourier-spectral method. It is shown that a biaxial substrate constraint has essentially no effect on the spinodally decomposed two-phase morphology which is primarily controlled by the cubically anisotropic elastic interactions. The asymmetry in the strain components along the [100] and [010] directions from the substrate constraint results in the preferential alignment along one of the two directions. In the particular case of a harder phase whose lattice parameter increases with composition, a tensile substrate constraint along the film plane leads to the alignment of two-phase microstructures parallel to the tensile direction. This article is based on a presentation made in the 2003 Korea-Japan symposium on the “Current Issues on Phase Transformations”, held at Marriott Hotel, Busan, Korea, November 21, 2003, which was organized by the Phase Transformation Committee of the Korean Institute of Metals and Materials.     

Phase equilibria and multiphase reaction diffusion in the Cr-C and Cr-N systems

The phase formation in the Cr-C and Cr-N systems was investigated using reaction diffusion couples. The carbides were prepared by reaction of chromium metal with graphite powder in the range 1143 to 1413 °C in argon atmosphere; the nitride samples by reaction of the metal with N₂ (≤31 bar) in the range 1155 to 1420 °C. While the carbide samples showed the three chromium carbide phases in form of dense diffusion layers between 1100 and 1400 °C, porosity occurred at temperatures above 1400 °C. The composition of the phase bands was measured by the means of electron probe microanalysis. For the Cr₂₃C₆ phase, a slightly higher C composition was found than given in the literature. In Cr-N diffusion couples both the δCrN_1−x and βCr₂N formed phase bands at T≥1150 °C. Because decomposition processes occurred upon cooling, quenching experiments were carried out in the range 1370 to 1420 °C at 31 bar N₂ to stabilize the phases. The EPMA investigations of the homogeneity ranges yielded a large increase of the homogeneity range for δCrN_1−x with increasing temperature. The nonmetal diffusion coefficients in all phases of both systems were calculated from layer growth and/or from concentration profiles. In δCrN_1−x the N diffusivity was found to be strongly dependent on the composition. The Vickers microhardnesses of the various phases were obtained by measuring the diffusion layers.     

Phase equilibria and multiphase reaction diffusion in the Cr-C and Cr-N systems

The phase formation in the Cr-C and Cr-N systems was investigated using reaction diffusion couples. The carbides were prepared by reaction of chromium metal with graphite powder in the range 1143 to 1413 °C in argon atmosphere; the nitride samples by reaction of the metal with N₂ (≤31 bar) in the range 1155 to 1420 °C. While the carbide samples showed the three chromium carbide phases in form of dense diffusion layers between 1100 and 1400 °C, porosity occurred at temperatures above 1400 °C. The composition of the phase bands was measured by the means of electron probe microanalysis. For the Cr₂₃C₆ phase, a slightly higher C composition was found than given in the literature. In Cr-N diffusion couples both the δCrN_1−x and βCr₂N formed phase bands at T≥1150 °C. Because decomposition processes occurred upon cooling, quenching experiments were carried out in the range 1370 to 1420 °C at 31 bar N₂ to stabilize the phases. The EPMA investigations of the homogeneity ranges yielded a large increase of the homogeneity range for δCrN_1−x with increasing temperature. The nonmetal diffusion coefficients in all phases of both systems were calculated from layer growth and/or from concentration profiles. In δCrN_1−x the N diffusivity was found to be strongly dependent on the composition. The Vickers microhardnesses of the various phases were obtained by measuring the diffusion layers.     

Cubic to tetragonal martensitic transformation in a thin film elastically constrained by a substrate

A 3-dimensional phase-field model is developed to describe the cubic to tetragonal martensitic phase transformation in a thin film attached to a substrate. Elasticity solutions are derived for both elastically anisotropic and isotropic thin films with arbitrary domain structures, subject to the mixed boundary conditions for stress-free and constrained states. The model is applied to an Fe-31%Ni alloy system. The nucleation process as well as the final domain structure strongly depends on the substrate constraint. At a smaller undercooling, the increased strain energy effect results in a lower volume fraction of martensite, a finer domain structure and a longer nucleation period. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, K orea on October 25–26, 2002.     

Computer simulation of spinodal decomposition in constrained films

The morphological evolution during spinodal decomposition of a binary alloy thin film elastically constrained by a substrate is studied. Elastic solutions, derived for elastically anisotropic thin films subject to the mixed stress-free and constraint boundary conditions, are employed in a three-dimensional phase-field model. The Cahn–Hilliard diffusion equation for a thin film boundary condition is solved using a semi-implicit Fourier-spectral method. The effect of composition, coherency strain, film thickness and substrate constraint on the microstructure evolution was studied. Numerical simulations show that in the absence of coherency strain and substrate constraint, the morphology of decomposed phases depends on the film thickness and the composition. For a certain range of compositions, phase separation with coherency strain in an elastically anisotropic film shows the behavior of surface-directed spinodal decomposition driven by the elastic energy effect. Similar to bulk systems, the negative elastic anisotropy in the cubic alloy results in the alignment of phases along 〈1 0 0〉 elastically soft directions.     

The effects of vacuum annealing on the film properties of titanium boride (TiBx) grown on (100)Si substrate

In order to investigate the effects of vacuum annealing on the properties of titanium boride films (TiBx) on a (100)Si substrate, TiBx/Si samples were prepared by the co-evaporation process and then annealed in the temperature range of 300≈1000°C. The interfacial reaction of TiBx/Si systems and the thermal stability of non-stoichiometric TiBx films (0≤B/Ti≤2.5) were investigated by means of sheet resistance, x-ray diffraction, transmission electron microscopy, x-ray photo-electron spectroscopy, and stress measurement. For TiBx samples with a ratio of B/Ti≥2.0, an apparent structural change is not observed even after annealing at 1000°C for 1 h. For samples with the ratio of B/Ti<2.0, however, there are two competitive solid phase reactions: the formation of a titanium silicide layer at the interface and the formation of a stoichiometric TiB₂ layer at the surface, indicating the salicide (self-aligned silicide) process. The sheet resistance and the film stress in the Ti/Si and TiB_x/Si systems are explained well by the solid phase reactions.     

Effect of bias on the As-deposited grain structure of Cr thin film

The film growth of Cr thin film by DC-magnetron sputter deposition was investigated by experimentally measuring the evolution of grain size distribution and by computing the film growth using Monte Carlo simulation. The as-deposited Cr thin film by sputter deposition typically grows in a columnar grain structure at the substrate temperature 260°C, which is far lower than 0.3 T_m. The stagnation of columnar grain structure does not occur in the case of no-bias condition up to the investigated film thickness of about 800 nm. However, the application of a negative bias of 200 V results in a stagnation of columnar grain structure at film thickness of about 50 nm and at the deposition temperature of 260°C. This is believed to arise from the fact that the mobility of ad-atoms is greatly enhanced and the Ar+ ions pin the grain boundary as a result of bias application. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformation of Nano-Materials,” organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University Seoul, Korea on October 25–26, 2002.     

Mechanical properties of the coating/substrate composite system: Nanostructured copper films on a LiF substrate

The study of a series of composite structures (CSs) such as CS-1, CS-2, and CS-3 being “soft film/soft substrate” systems revealed much new information on the mechanical properties of these materials. The general and distinctive properties of CSs both within the series of СS-1, CS-2, and CS-3 and the properties of raw materials of Cu and LiF are considered. It is found that the deformation process passes through three main phases in a wide range of loads at the nanomicroindentation of Cu/LiF CSs: (1) when β = h/t < 0.5 (h is the indentation depth, t is the coating thickness) the plastic deformation is mainly concentrated in the film, and only a small elastic deformation can take place in the substrate; (2) at β ≈ 1.0 the deformation occurs in the film and in the interface zone; (3) when β > 1.0 the plastic deformation extends into the substrate bulk, capturing all the typical levels of the system (film–interface zone–substrate) naturally becoming more complex as the load increases. It is shown that the “film/substrate” CSs are complex systems with their highly individual properties even possessing the same chemical composition with the same production method, differing by only one parameter (the film thickness t).     

Effect of heat treatment and plastic deformation on the structure and elastic modulus of a biocompatible alloy based on zirconium and titanium

Transmission electron microscopy and X-ray diffraction analysis have been used to study the phase composition and structure of an IMP-BAZALM (Zr-31Ti-18Nb (at %)) biocompatible medical alloy depending on the heat-treatment conditions. An interrelation between the values of the electron concentration of phases normalized to the volume (e/a _norm = e/a _alloy(ν_β/ν_phase)) and the phase composition of the alloy has been found to exist. It has been established that the appearance of the α″ and ω phases in the structure of the alloy leads to an increase in the modulus of elasticity. The greatest increase in the modulus is observed under the conditions corresponding to the formation of the ω phase.     

Diffusion and phase transformation on interface between substrate and NiCrAlY in Y-PSZ thermal barrier coatings

NiCrAlY/Y₂O₃-Y-PSZ (yttria-partially stabilized zirconia) thermal barrier coatings were developed on a superalloy (Ni-10Co-9Cr-7W-5Al, wt.%) surface. The superalloys were first coated with a bond coat of Ni-19Cr-8Al-0.5Y (wt.%) alloy that was deposited by low-pressure plasma spraying and then covered with a top coat of ZrO₂-8wt.%Y₂O₃ by air plasma spraying. The microstructure near the interface was analyzed using an optical microscope, a scanning electron microscope, microhardness measurements, and x-ray diffraction, and the phases of composition were measured using an electron probe microanalyzer after exposure at 1100°C for different times in air or a vacuum. The reaction processes also were simulated using diffusion-controlled transformation (DICTRA) software in which diffusion was considered as being only the γ phase, and the γ′ phase was treated as spheroidal particles in γ. From the authors’ results, it can be concluded that a γ′-phase layer is observed at the interface between substrate and bond coat, and its thickness increases with increasing exposure times in air at 1100 °C. This layer showed good cohesion with the substrate and bond coat. It can also be concluded that the formation of the γ′-phase layer can be predicted from DICTRA simulation. The simulation also shows the same trend of the composition profiles as experimental data.     

Improvement of Electrochemical Surface Properties in Steel Substrates Using a Nanostructured CrN/AlN Multilayer Coating

Improvement of corrosion properties on AISI D3 steel surfaces coated with [CrN/AlN] _n multilayered system deposited for various periods (Λ) via magnetron sputtering has been studied in this work exhaustively. For practical effects compared were the latter properties with CrN and AlN single layers deposited with the same conditions as the multilayered systems. The coatings were characterized in terms of crystal phase; chemical composition, micro-structural, and electrochemical properties by x-ray diffractometry, energy dispersive x-ray, Fourier transforming infrared spectroscopy, atomic force microscopy, scanning electron microscopy, Tafel polarization curves, and electrochemical impedance spectroscopy. Corrosion evolution was observed via optical microscopy. Results from x-ray diffractometry analysis revealed that the crystal structure of [CrN/AlN] _n multilayered coatings has an NaCl-type lattice structure and hexagonal structure (wurtzite-type) for CrN and AlN, respectively, i.e., it was made non-isostructural multilayered. The best behavior was obtained by the multilayered period: Λ = 60 nm (50 bilayers), showing the maximum corrosion resistance (polarization resistance of 1.18 KΩ, and corrosion rate of 1.02 mpy). Those results indicated an improvement of anticorrosive properties, compared to the CrN/AlN multilayer system with 1 bilayer at 98 and 80%, respectively. Furthermore, the corrosion resistance of steel AISI D3 is improved beyond 90%. These improvement effects in multilayered coatings could be attributed to the number of interfaces that act as obstacles for the inward and outward diffusions of ion species, generating an increment in the energy or potential required for translating the corrosive ions across the coating/substrate interface. Moreover, the interface systems affect the means free path on the ions toward the metallic substrate, due to the decreasing of the defects presented in the multilayered coatings.     

Charge-discharge induced phase transformation of RuO<Subscript>2</Subscript> electrode for thin film supercapacitor

We report on the preparation of an all solid-state thin film micro-supercapacitor using RuO₂ electrode film and LiPON electrolyte film on a Pt/Ti/Si substrate with dual target dc and rf reactive sputtering. Room temperature charge-discharge measurements based on a symmetrical RuO₂/LiPON/RuO₂ structure clearly demonstrated the cyclibility dependence of the RuO₂ electrode on the microstructure. Using both glancing angle X-ray diffraction (GXRD) and transmission electron microscopy (TEM) analysis, it was found that the characteristics of the thin film supercapacitor are dependent on the microstructure of the RuO₂ film. In addition, high-resolution electron transmission microscopy (HREM) analysis after cycling demonstrates that the interface layer formed by interfacial reaction between the LiPON and RuO₂ acts as the main factor in the degradation of the performance of the thin film micro-supercapacitor. This article is based on a presentation made in the 2002 Korea-US symposium on the “Phase Transformations of Nano-Materials”, organized as a special program of the 2002 Annual Meeting of the Korean Institute of Metals and Materials, held at Yonsei University, Seoul, Korea on October 25–26, 2002.     

Experimental investigation of the Tb-Mg phase diagram

The Tb-Mg alloys were studied in the range 0 to 100 at.% Mg, by using X-ray powder diffraction, optical and scanning electron microscopy, electron probe microanalysis (EPMA), and differential thermal analysis (DTA). The following intermediate phases were identified and their crystal structures confirmed: TbMg (cubic,cP2 CsCl type), TbMg₂ (hexagonal,hP12-MgZn₂ type), and TbMg₃ (cubic,cF16-BiF₃ type). Two phases, moreover, were confirmed or identified in a composition range very close to 83 at.% Mg: χ₁ phase (cubic,cF440-GdMg₅ type) and χ₂ phase (cubic,cI58-αMn type). All the phases show peritectic formation with the possible exception of the χ₂ phase. The following phase equilibria were also determined: a eutectic reaction at 530 °C and 90.5 at.% Mg and a eutectoidal decomposition of the (βTb) phase at 670 °C and ∼28 at.% Mg.     

Characteristics of IZSO films deposited by a co-sputtering system

In−Zn−Sn−O films were deposited on a polycarbonate (PC) substrate by a magnetron co-sputtering system using two cathodes (DC, RF) without substrate heating. Two types of ITO targets (target A: doped with 5 wt.% SnO₂, target B: doped with 10 wt.% SnO₂) were used as an In−Sn−O source. The ITO and ZnO targets were sputtered by DC and RF discharges, respectively, and the composition of the In−Zn−Sn−O films was controlled via the power ratio of each cathode. In the case of ITO target A, the lowest resistivity (4.3×10⁻⁴ Ωcm) was obtained for the film deposited at the RF power (ZnO) of 55W. In the case of ITO target B, the lowest resistivity (2.9×10⁻⁴ Ωcm) of the film was obtained at the RF power (ZnO) of 30W, which was attributed to the increase in carrier density. Hall mobility decreased with increasing carrier density, which could be explained by the increase in ionized impurity scattering.     

The characteristics of gold films deposited on ceramic substrate

A large quantity of gold (approximately 10 tonnes yearly) is consumed, all over the world, just to decorate ceramic and glassware. Due to their advanced chemical stability gold films are used for different high technology applications. The technologies for obtaining the best “liquid bright gold” were intensively studied, but the quality of the decor coatings (films) were empirically assessed. We proposed a scientific investigation of the characteristics of gold films, deposited on ceramic substrates, from “liquid bright golds”. The composition of the film has been determined by EDS (Energy Dispersive X-ray Spectrometry). The distribution of the elements was determined at the surface of the film and in cross-section. The surface distribution of the elements was uniform. The diffusion process of the film into substrate and the migration of the substrate elements at the interface region and into the film have been highlighted.     

Substrate Effect on the High Temperature Oxidation Behavior of a Pt-modified Aluminide Coating. Part II: Long-term Cyclic-oxidation Tests at 1,050 °C

This second part of a two-part study is devoted to the effect of the substrate on the long-term, cyclic-oxidation behavior at 1,050 °C of RT22 industrial coating deposited on three Ni-base superalloys (CMSX-4, SCB, and IN792). Cyclic-oxidation tests at 1,050 °C were performed for up to 58 cycles of 300 h (i.e., 17,400 h of heating at 1,050 °C). For such test conditions, interdiffusion between the coating and its substrate plays a larger role in the damage process of the system than during isothermal tests at 900, 1,050, and 1,150 °C for 100 h and cyclic-oxidation tests at 900 °C which were reported in part I [N. Vialas and D. Monceau, Oxidation of Metals 66, 155 (2006)]. The results reported in the present paper show that interdiffusion has an important effect on long-term, cyclic-oxidation resistance, so that clear differences can be observed between different superalloys protected with the same aluminide coating. Net-mass-change (NMC) curves show the better cyclic-oxidation behavior of the RT22/IN792 system whereas uncoated CMSX-4 has the best cyclic-oxidation resistance among the three superalloys studied. The importance of the interactions between the superalloy substrate and its coating is then demonstrated. The effect of the substrate on cyclic-oxidation behavior is related to the extent of oxide scale spalling and to the evolution of microstructural features of the coatings tested. SEM examinations of coating surfaces and cross sections show that spalling on RT22/CMSX-4 and RT22/SCB was favored by the presence of deep voids localized at the coating/oxide interface. Some of these voids can act as nucleation sites for scale spallation. The formation of such interfacial voids was always observed when the β to γ′ transformation leads to the formation of a two-phase β/γ′ layer in contact with the alumina scale. On the contrary, no voids were observed in RT22/IN792, since this β to γ′ transformation occurs gradually by an inward transformation of β leading to the formation of a continuous layer of γ′ phase, parallel to the metal/scale interface.     

Evaluation of Ni film interfacial energy release rate on titanium and stainless steel substrates

An expression including the effect of residual stress on the interfacial energy release rate is proposed for peeling experiments according to the energy-balance argument. The influence of residual stress on the external work is also contained in the expression. Two numerical methods are employed to evaluate the values of the work expenditureG _db , which is the actual energy dissipated during bending of the peel arm near the peel front. The peeling method is employed to test the interfacial energy release rates,G, for Ni films on Titanium and stainless steel substrates. The results indicate that the value ofG for Ni films on stainless steel substrate is about 5.47–6.03 N/m, while 5.23–6.71 N/m for Ni films on titanium substrate; the interfacial energy release rates,G, do not depend on the residual stress in film, film thickness nor peel angle. The effect of residual stress in film on peel strengthP/h is also discussed.     

Effect of dysprosium and cobalt on the temperature dependence of magnetization and phase composition of a material of the Nd-Dy-Fe-Co-B system

Sintered magnetically hard materials with composition (Nd_1−x Dy_x)_13–16(Fe_1−y Co_y)_resAl_0.5−1B_7–9 (x = 0.11–0.71, y = 0.19–0.34) are investigated. The role of cobalt and dysprosium in the variation of magnetic properties of the material (residual induction, coercive force) is studied. The phase composition of the material and its effect on magnetic properties are determined. Amethod for computing the induction temperature coefficient of a material on the basis of published data on the physical characteristics (the Curie temperature, the density, the molecular mass) of the main magnetic phase (Nd, Dy)₂(Fe, Co)₁₄B is presented. __________ Translated from Metallovedenie i Termicheskaya Obrabotka Metallov, No. 4, pp. 3–10, April, 2007.     

On the Microstructure of the Initial Oxide Grown by Controlled Annealing and Oxidation on a NiCoCrAlY Bond Coating

Different pre-annealing and pre-oxidation treatments were conducted on a dual phase γ+β Ni–21Co–18Cr–22Al–0.2Y (at.%) bond coating for 1 hr at 1373 K (i) with or without a native oxide upon heating, (ii) in two different atmospheres upon heating, and (iii) under various oxygen partial pressures (pO₂) in the range of 0.1–10⁵ Pa during oxidation. The chemical composition, structure, morphology and phase constitution of the resulting oxide layers were investigated using a range of analytical techniques. It is found that the exclusive formation of a continuous α-Al₂O₃ layer without the simultaneous formation of NiAl₂O₄ spinel was promoted for oxidation at low pO₂. The formation of metastable θ-Al₂O₃ was suppressed for a low fraction of the β phase, coupled with a high fraction of segregated Y at the initial bond coat surface. Initial Y segregation and incorporation of Y₂O₃ and Y₃Al₅O₁₂ within the developing oxide layer was promoted in the absence of a native oxide and for heating in an inert atmosphere. The development of protrusions (i.e. pegs) at the oxide/coating interface, as a result of the incorporation of internal Y₂O₃ precipitates by the inward growing oxide layer, was most pronounced upon heating in an inert atmosphere, followed by oxidation at an intermediate pO₂.